"Five Daesh suicide bombers... Wearing explosive belts attacked a police and army base at dawn in the Al-Masafi district," the town's mayor Mohammad Mahmoud told AFP, using an Arabic acronym for the group.
"Seven members of the security forces were killed and six wounded" in the ensuing firefight, he said.
Interior ministry spokesman Brigadier General Saad Maan confirmed the toll and said the "five suicide terrorists" were also killed.
Baiji was the scene of some of the longest-running battles between IS and Iraqi forces.
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The town lies at a major crossroads and its recapture by Iraqi security forces was seen as key to preparing the ground for offensives in Anbar province and later Mosul.
The town and its nearby oil refinery, Iraq's largest, were recaptured in late 2015 with air support from a US-led coalition and a host of tribal groups and militias.
The latest attack comes as Iraqi warplanes pound IS positions in preparation for a ground assault on the town of Tal Afar, west of Mosul.
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